I was looking through a Wikipedia Page titled "iOS 4 Version History". Looking under iOS 4 I found a feature called "custom dictionary." I didn't know this feature didn't even exist until I was researching about iOS 4 features. I went to Setting>General>Keyboard but there was no Custom Dictionary. Playing around with the keyboard I found that by enabling Japanese Ten Key Keyboard you enable custom dictionary. Then when you tap the back button and you go back to the keyboard menu, on the bottom you find the custom dictionary feature. When you want to add a word that doesn't appear correctly in the iPhone's dictionary, and your tired of seeing the little red line on the bottom you go to "edit user dictionary" tap "word" and then you type the word and save it!